The coordinated movements of speech demand the temporal-spatial interaction of several interdependent motor subsystems. As such the control required to produce these coordinate movements should reflect the flexibility and complexity involved in this uniquely human motor act. Based upon our research and that of others, it appears that the orofacial motor actions for speech are under continuous afferent-dependent influence through operation of both open-loop, feedforward and closed-loop, feedback control mechanisms. Despite recent advances, our understanding of the detailed operation of these sensorimotor mechanisms involved in the control of motor speech currently is rudimentary. For example, while it is apparent that speech movement coordination and control involves multiple sensorimotor actions, the moment-to-moment conditions under which these various mechanisms are operating remains largely undefined. The investigations that are proposed here are an attempt to determine the variables underlying the differential utilization of these multiple processes in the control of the orofacial complex for speech. This research will exploit recent advances in both measurement techniques (computer implemented movement and EMG analyses) and technology (DC brushless torque motor) in the area of speech motor control. The contribution of afferent information to the control of speech movements will be evaluated by applying unanticipated loads (perturbations) to the upper lip, lower lip, or jaw during speech tasks requiring the coordinated actions of these orofacial structures. The control processes and potential pathways underlying loads compensation will be investigated through analyses of the latency, loci, and magnitude of the compensatory responses. From these determinations, it should be possible to determine if and when short latency brain stem reflexes are utilized in the control of orofacial movements as well as when speech motor control processes are dominated by suprabulbar control mechanisms. Finally, through the use of select subjects with movement disorders (Parkinson's disease and cerebellar ataxia) it will be possible to refine our hypotheses concerning the potential neural structures involved in speech motor control as well as more precisely defining the speech motor aberrations associated with these pathological conditions.